Copies the source email under the userId specified in the request body and returns the complete newly created email.

Validation and behavior:

• userId is required, must not exceed 255 characters, and must match ^[A-Za-z0-9_-]+$.
• userId must resolve to the owner of the source email under the current application. This endpoint does not create an end user.
• name is optional and must not exceed 64 characters. When omitted, null, empty, or blank, the server uses the source email name plus _copy, truncated to 64 characters when necessary.
• subject, summary, snapshot, html, settings, extra, component configuration, and source information are copied from the source email and cannot be overridden by this request.
• The copied email is independent from the source email and receives a new external id.